1. Field of the Invention
This invention relates to methods for the continuous cooling of wire rod of low carbon steel (C.ltoreq.0.4% and preferably C.ltoreq.0.15% ).
On discharge from a rod mill, wire rod is subjected to a controlled cooling operation whose operating parameters depend on the final properties with which it is desired to provide the rod.
Taking into account the composition of the rod and the final properties with which it is desired to provide the rod, it is possible to vary to a considerable extent the duration, the number, the type, and the intensity of the various stages of the complete cooling process.
The large size of the plants for carrying out the required cooling is a further aspect of the problem, and in particular their dimensions in the longitudinal direction, which may considerably affect the capital required to construct them.
Consequently an improvement of any one of the stages of a rod cooling process may have an effect on the quality of the rod, the time required to completely cool the rod, the size of the corresponding plants, and therefore on both their operating and first installation costs and on the homogeneity of the mechanical characteristics.
There are at present numerous method of subjecting wire rod, on discharge from the mill, to a specific cooling sequence, often specifically designed for the required rod quality.
In the case of extra mild steel rod, it is essentially attempted to obtain an elastic limit (E), a tensile strength (R), and an E/R ratio which are as low as possible. It is therefore necessary, in the finished product, for the ferritic grain to be coarse and the amount of C in supersaturation in the ferrite to be minimal.
According to a known method, on discharge from the last stand of the rolling mill, wire rod of low carbon steel is subjected to cooling by spraying with water until it reaches a temperature of between 850.degree. C. and 800.degree. C., and then disposed in loose turns on a horizontal conveyor, where it is further cooled by still air then blown air to a temperature in the range of 200.degree. C. to 300.degree. C., and finally coiled at this temperature for delivery. It can therefore be seen that, in this method, a relatively intense and rapid cooling stage in the temperature range favourable to precipitation follows a holding stage at a high temperature which is relatively long and that consequently there has been little carbide precipitation when the rod has reached the coiling temperature. An operating method of this type does not appear to lead specifically to rod of a particularly mild grade, as there is no overaging stage following cooling of this type.
According to a further known method which may be applied advantageously to the manufacture of mild steel wire, on discharge from the last stand of the rolling mill the wire rod, which is again pre-cooled, is disposed in loose turns on a first conveyor, at a temperature similar to that given above; cooling is carried out slowly in air on this conveyor until the allotropic austenite to ferrite transformation is partially achieved. This stage is followed by quenching of the rod in a bath of boiling water in which the allotropic transformation is completed. The rod is finally subjected to an overaging operation at a mean temperature of 450.degree. C. for at least one minute and a product which is suitable for normal use as mild steel rod is thus obtained.